1. Field of the Invention
The present invention relates to a carbon sensor electrode for use in various sensors such as electrochemical detectors, sensors for environment analysis, sensors for pathological examinations, and probe electrodes for detection of in vivo conditions. It also relates to a process for producing the carbon sensor electrode.
2. Description of the Prior Art
Among major fields of electrochemical measurement is simple rapid analysis relying for detection on sensors that have shown prompt progress in recent years. Since this method is highly selective and permits high sensitivity measurement, it has found extensive use in the analysis and evaluation of clinical samples or environmental samples containing minute quantities of ingredients to be determined and having numerous compounds coexistent therein.
In recent years, it has become crucial to obtain information on such particular substances in vivo and in situ locally in a biological system by use of a sensor electrode which can detect these substances with high sensitivity and rapidity. Acquisition of to that information requires the use of an easily producible disposable electrode, or the use of a sensors from which a fresh electrode surface can be easily obtained and which can selectively measure a particular substance in vivo.
Such measuring electrodes which have been studied and put to practical use include pH meters which measure hydrogen ions; ion sensors which detect inorganic ions such as sodium ions; sensors comprising carbon fibers or glassy carbon chemically modified with complexes or organic matters on the surfaces thereof; various enzyme sensors such as glucose sensors prepared by mixing enzymes such as glucose oxidase with the paste of a carbon paste electrode, and coating the mixture with an ion permeable membrane; and biosensors holding metabolites from immune mechanisms.
However, most sensors including chemically modified sensors, enzyme sensors and biosensors, except for pH meters and inorganic ion sensors, are defective in that they cannot withstand long-term use and their lives are short. Thus, a desire has been strong for the development of long life sensor electrodes preferred for use in sensors excluding pH meters and inorganic ion sensors.
The requirements for sensor electrodes include, for instance,
(i) a large potential window and a low blank current, PA1 (ii) a highly reproducible, repeatable reaction of a particular substances, PA1 (iii) an active electrode reaction, PA1 (iv) no differences among individual electrode characteristics, PA1 (v) little content of impurities, thus not interfering in an electrode reaction, and PA1 (vi) ease of handling and pretreatment. PA1 1. It provides a cell-scale carbon microsensor electrode which can apply an electric current, an electric voltage or a mechanical stimulus to a living system. PA1 2. It does not poison the measuring system (if it remains in vivo, it is safe), and it can be used for testing foods. PA1 3. It has a mechanical strength enough high to be stuck into the body or food, permitting the electrochemical detection of a very fine (a very small amount of) portion. PA1 4. It has minimally fluctuating characteristics, offers reproducible data, and enables reliable measurement. PA1 5. It does not require special pretreatments, and polishing is sufficient for its stable measurement of an electrode reaction. PA1 6. It is low-priced and disposable. PA1 (1) Carbon compatible with the body constitutes an elect rode. PA1 (2) The carbon material has high activity as an electrode. PA1 (3) A reactant such as an enzyme can be held active in the sensor. PA1 (4) A used electrode portion can be snapped or cut off. PA1 (5) The price is low enough for disposable use.
We, the present inventors, previously found that a probe electrode using the lead of a mechanical pencil which is a thin rod of carbon (Japanese Patent Application Laying-open No. 250854/1989) could be used as a material for a sensor electrode fulfilling most of the above-mentioned requirements, and that some carbon fibers could be used in measuring electrodes. As is well known, that mechanical pencil lead is a composite carbonaceous material comprising naturally occurring graphite and an organic binder.
We attempted to solve the aforementioned problem of life with the conventional sensors by using the composite carbonaceous material. Prior to the filing of the present application, we proposed a sensor preferred as a chemically modified sensor, an enzyme sensor or a biosensor (Japanese Patent Application Laying-open No. 18928/1993). Since micropores are formed during the production of the composite carbonaceous material, this previously proposed sensor takes advantage of the micropores to achieve increased conductivity. It also has controlled pore sizes, thereby making a sensor substrate usable for long periods. In this sensor, moreover, the surface and internal communicating micropores of a porous carbon thin wire which is the sensor substrate are impregnated, adsorbed or chemically modified with a reactant such as an enzyme, a metal complex, an organic substance or a metabolite, whereby the desired reactant is held inside or on the surface.
This electrode is a carbon sensor electrode capable of detecting particular substances, and has the following characteristics:
However, this electrode is disadvantageous in that its poor carbon wettability may pose difficulty in impregnating a reactive substance into the resulting micropores.
The living system where the carbon sensor electrode is used for measurement contains large amounts of adsorbable organic substances such as proteins and lipids in a coexistent fashion. Once adsorbed to the carbon surface of the sensor, these organic matters are not easily removable unless oxidized. If the carbon sensor electrode is to be reused, therefore, care should be taken to discard the electrode portion after each use, or to break off the used electrode portion. In clinical examinations, the reuse of the same sensor that was used on other samples may cause errors for the above reason. Thus, a carbon sensor electrode having the used electrode portion disposable is desired. The requirements for a carbon sensor and a carbon sensor electrode of a construction with disposable characteristics are as follows:
A disposable carbon sensor electrode should fulfill at least the above five requirements. For this purpose, a carbon sensor electrode is desirable in which the desired reactant such as an enzyme, a metal complex or a metabolite is always easily made to exist on the surface of the electrode and which can be produced economically. In order for the carbon sensor electrode to be usable for long periods, it is important that the requirement (3), in particular, be fulfilled.
Furthermore, there are some reactants which become deactivated by the oxygen in the air. In this case, to solve this problem and have the carbon sensor ready for measurement where necessary, the important task is to prepare a redox type carbon sensor electrode whose potential is kept to be oxidation or reduction potential so that the reactive substance may be kept active.
In addition to the carbon sensor electrode using the composite carbon material (lead of a mechanical pencil), a preferred sensor electrode is conceivable which is a commercial carbon paste electrode prepared by mixing fine carbon particles with a liquid. Such a commercially available carbon paste electrode has a mixture of carbon particles for electrode and a liquid held at the tip of the electrode, or may have a reactant additionally mixed. Some of such carbon paste electrodes may be used as disposable sensors. The carbon paste electrode, however, must be handmade for each use, thus often causing delicate differences in the mixing state or in the electrode surface every time it is prepared.
The object of the present invention, therefore, is to provide the construction of a carbon sensor electrode satisfying the above-described requirements, and a process for producing the carbon sensor electrode.